In order to make efficient use of the microwave radio spectrum, state-of-art digital point-to-point radio systems employ highly bandwidth-efficient modulation techniques. At present, certain known commercial systems employ 128 QAM signal constellation, nevertheless some 512 QAM prototypes are currently being field-tested. For the sake of clarity it is to be noted that “QAM” stands for Quadrature Amplitude Modulation which is a well known amplitude modulation scheme in which amplitude modulation is performed by two separate signals of two sinusoidal carriers having the same amplitude and frequency but being in phase quadrature, the modulated signals are then added for transmission on a single channel.
As the number of the points of the signal constellation grows, the system becomes more sensitive to all types of linear and nonlinear signal distortion. A particularly critical issue in bandwidth-efficient QAM systems is the phase noise (PN) of the local oscillators (LO) which are used to convert the modulated signal from IF to RF and vice versa. Together with other noise-like degrading effects, LO—phase noise (LOPN) gives rise to a constant irreducible bit error rate (BER) independently from the power of the received signal.
There are two ways known in the related art to face the problem of LOPN. The first known method is to achieve low noise local oscillators. However, the higher the radio frequency is, the more difficult it is to design and to produce a local oscillator with low phase noise. The second known solution is to choose a demodulation process which is somehow non sensitive towards PN.
Consequently the first solution may become considerably expensive, taking into account in particular the fact that at present the radio frequencies that are being used are growing higher and higher in value (e.g. up to 90 GHz). Nevertheless, the second solution can be implemented with phase locked loop (PLL). In fact the PLL principle has been successfully used for decades for tracking the carrier phase. There is plenty of literature available to the public on PLL design and techniques. As a non limiting example reference is made to Floyd M. Gardner, “Phaselock Techniques”, Second Edition, John Wiley & Sons.
In the context of the above problem, the bandwidth of the PLL is the main parameter to take under consideration. The wider the bandwidth is, the better the demodulator ability becomes in order to track LOPN.
However, there are two particular limits in achieving wide bandwidth for the PLL. One such limitation is the introduction of an additional source of phase noise which is due to the phase estimator error introduced by the modulated signal itself; this additional phase noise—that depends also from the quality of the received signal level and from the amount of the additive white gaussian noise (AWGN)—must, as much as possible, remain much lower than the original one. The second limit is the loop electrical delay. It turns out that a long loop electrical delay makes it impossible to go beyond a certain bandwidth, because the phase transfer function of the PLL approaches levels close to instability.
From the above discussion it is clear that in order to reach an optimum design of the PLL, its bandwidth and its transfer function depend on the amount of LOPN as well as on the amount of the AWGN.
The “standard” solution for achieving the above objectives would be to design a PLL with a fixed bandwidth as wide as possible. However, taking into account that the optimum bandwidth of the PLL depends on the signal to noise ratio of the received signal, then the choice of a fixed bandwidth cannot be considered an “optimum” solution because the signal to noise ratio can change depending on the circumstances.